Barium silicate doped with lead (BaSi.sub.2 O.sub.5 :Pb) is widely used in speciality fluorescent lamps, particularly for tanning applications.
While the spectral properties of this phosphor make it well suited for fluorescent lamp tanning applications, the one major disadvantage in its use is that it contains toxic elements which may ultimately leach into the environment as waste waters during lamp manufacturing or after the disposal of expended lamps after their useful life. As the demand for these lamps increases, there are environmental concerns due to the presence of barium and lead in the phosphor. Methods and phosphors which reduce or eliminate these environmental concerns are highly desirable.
U.S. Pat. No. 4,691,140 to Sakakibara et. al. describes a glass bulb with a BaSi.sub.2 O.sub.5 :Pb phosphor coating layer where alumina particles having an average diameter of 0.01 to 0.1 .mu.m act as a protective coating between the phosphor and the glass. As set forth, the coating reduces interaction with the glass and subsequent reduction in luminous efficiency of the fluorescent lamp. The coatings are prepared by baking a dispersion of alumina with a binder in an organic solvent after applying the dispersion to the inside of a bulb. Although the coating is set forth as having enhanced luminous efficiency, such a coating does not address environment concerns.
U.S. Pat. No. 4,950,948 to Peters, et. al. relates to a manganese zinc silicate phosphor which can be coated with a continuous, conformal, nonparticulate aluminum oxide coating and annealed to improve its maintenance in a fluorescent lamp. The aluminum oxide coating is formed by a chemical vapor deposition process within a fluidized bed of the phosphor powder using trimethyl aluminum and oxygen as the reactants. The purpose of the annealing of the coated phosphor, as described in U.S. Pat. No. 4,803,400 to Peters and Hunt, is to prevent hydration/solubilization of the oxide coating. It is an essential step to stabilizing the coating since water is the principal agent in the destruction of the alumina (see column 3, lines 55-56 of that patent). Unfortunately, as described in '948, the thermal annealing process which bonds the alumina coating to the phosphor particle has the undesirable side effect of promoting the interdiffusion of chemical species which make up the coating and phosphor substrate (see column 10, lines 52-56). Even the earliest patent which first describes the annealing process ('400) refers to the migration of zinc via diffusion through the coating as determined by XPS, X-ray Photoelectron Spectroscopy, which is a powerful technique for determining the chemical composition of the surface of an annealed alumina coated zinc silicate phosphor (see Example 2, column 5, lines 40-43).
U.S. Pat. No. 4,999,219 to Klinedinst and Gary describes another method of producing a continuous, conformal, non-particulate aluminum oxide coating which employs aluminum isopropoxide as the precursor in a fluidized bed. XPS elemental surface analysis of the alumina coated zinc silicate particles show no zinc, silicon, or manganese. However, upon annealing, zinc and manganese in substantial quantity are detected. This has been attributed to these elements diffusing from the surface of the phosphor contacting the alumina coating during the anneal. Subsequent detailed XPS analyses have shown that tungsten, which is originally used in the synthesis of the phosphor, also migrates through the alumina coating. Patent '219 also notes that "similar results were obtained with alumina-coated ,cool-white, (i.e., calcium halophosphate) phosphor". Therefore, it appears that elements of base phosphors are highly mobile during the annealing process used to bond the alumina coating to the phosphor substrates and can easily migrate to the surface of the alumina coated phosphors.
To be effective in preventing the leaching of elements from the phosphor of environmental concern, it is desirable that the core elements of the phosphor do not migrate through to the outer surface of the encapsulating coating used to protect it.
Heretofore, environmental concerns have concentrated on the extraction of mercury from lamps after their useful life. Industry and the prior art have not focused on environmental concerns relating to lamps of the above nature containing lead and barium.